Coal-fired combustion chambers, for example those used in utility plants to generate electricity, require the efficient delivery of pulverized coal. Bowl mills of a known type are used to pulverize the coal. In prior art systems, it is known to use a coal-pulling fan, known as an exhauster fan, between the bowl mill pulverizer and the combustion chamber, creating an air flow which draws the pulverized coal from the mill into the fan through a generally circular inlet, the exhauster fan radially directing the coal into a conduit extending to one or more nozzles disposed in the vicinity of the combustion chamber. The fans can employ a conical diverter cap on their hub, which serves to radially divert the coal into the rotating fan blades.
These coal-side exhauster fans are sometimes referred to as "dirty side" fans, because they are positioned in the middle of the coal flow between the mill and combustion chamber.
Coal-side exhauster fans are also known to employ what are known as "whizzer" blades mounted on the leading edges of the primary fan blades. The whizzer blades emanate radially from a whizzer disk and ring arrangement mounted coaxially with the fan and the inlet. The purpose of the whizzer blade and supporting structure is ostensibly to prevent heavier particles of coal from settling out or being trapped between the fan and the fan housing, where it causes wear on the housing.
A serious disadvantage with such whizzer blade structure is that it adds significantly to the weight of the fan, and consequently to the power required to drive the fan. In a continuously-running large utility application, such power costs can be considerable. Moreover, it is this inventor's belief that the whizzer blades are largely ineffective in achieving their stated purpose of preventing the settling out of heavier coal particles between the fan blades and the housing.
One attempt to improve coal flow through such an exhauster fan as described above has been to provide a shallow, sharply-angled, venturi-shaped cone in the exhauster fan inlet to direct the incoming coal-air mixture toward the center of the exhauster fan rather than into the whizzer blades. This experiment was deemed a failure, however, because the steep angle of the cone impeded, rather than helped, flow through the inlet. Moreover, the short length of the cone (substantially less than the axial length of the exhauster fan inlet), its correspondingly sharp transition angle (necessary to direct coal into the whizzer ring extending into the inlet), and a significant axial gap between the cone outlet and the whizzer ring inlet caused significant eddy effects, carrying coal flow backward into the gap between the cone and the exhauster inlet and creating a substantial pressure drop which actually impeded flow.